Gravure, flexography and offset printing generally are high speed printing processes that result in high quality printed images. The high speed results from the ‘stamping’ nature of these processes, where a printing surface or printing plate has a printing pattern formed on it that, when inked and transferred to a printing substrate, forms a print image. After the inking process, the ink is transferred from the print image to a printing substrate. High quality prints are achieved due to the use of high viscosity inks with high pigment loading and due to printing at high pixel or ink dot density. The printing plate and printing pattern may take different forms depending upon the printing process in which they are used.
In gravure printing, the printing plate, which may actually be a cylinder used in a rotary printing press, has wells formed in the areas needed to form the desired image. The surface receives the ink and a blade, such as a doctor blade common to printing systems, removes any excess, so that the ink is captured only in the wells. Varying the depth of the wells achieves images with better gray-scale. The system then applies a high contact pressure to the printing surface against a printing substrate to transfer the ink to the printing substrate. A printing substrate may include paper, transparency, foils, plastics, or an impression roller, etc. Generally, due to the high contact pressure necessary, gravure printing processes print to paper or relatively sturdy substrates.
In flexographic printing, the process has many similar steps, except that the system raises the wells, or inked pixels, above the surface, similar to a rubber stamp. Ink transfer occurs with less force, so the process can use ‘softer’ printing plates made out of rubber or other elastomers more appropriate for printing substrates or media other than paper, such as transparencies, foil, labels, plastic, etc. For purposes of the discussion here, the wells of gravure printing, the inked pixels above the surface for flexographic printing, or any other region on the surface of the printing plate that is defined to form a printing pattern will be referred to as ‘defined regions.’
In either of the above examples, as well as many others, the term ‘printing plate’ means the surface upon which the print pattern is formed and is initially inked. For gravure printing, it may be a metal cylinder that is engraved with the recesses to capture ink, for flexography it may be a rubber cylinder or partial cylinder that has raised areas for accepting ink. In other applications, such as offset printing, the print image may be formed on the printing page by areas that accept ink and areas that do not.
Another possible printing system would be screen printing. In screen printing, a screen of highly porous, finely woven material is coated in areas in which ink is not desired and left porous where ink is desired. A squeegee or rubber blade pushed ink through the porous portions of the screen onto the substrate. In this instance, the printing plate would be the screen and the printing image is the image formed by the areas of porosity of the screen.
Both gravure and flexographic printing generally require etching of a master plate using wet processing involving various chemicals with drying steps that takes a relatively long time. Dry processes are desirable, but current techniques generally require a powerful laser to etch the plates.